This application claims benefit of U.S. Provisional 60/326,704 filed Oct. 3, 2001.
Cellulose is the most abundant biorenewable material and cellulose-derived products have been used in all cultures from the most primitive to highly developed modern technological society. Apart from the use of unmodified cellulose-containing materials (for example wood, cotton), modern cellulose technology requires extraction and processing of cellulose from primary sources using techniques that have changed very little since the inception of the modern chemical industry.
Cellulose and its derivatives can be substituted as a source for a number of chemicals. For example, petroleum feed stocks can be substituted with cellulose to prepare polymers for applications in paints, plastics and other formulation materials. Cellophane is prepared through the intermediacy of viscose that is dissolved, and then regenerated, whereas chemical dissolution typically incorporating derivatization such as ester or ether formation yields a wide range of modern materials.
The primary chemistry for transformation of cellulose is esterification; cellulose esters have important large-scale applications in the paper industry, for the preparation of fibers and textiles, as well as polymers and films. Mixed esters such as acetate/propionate or acetate/butyrate are used in plastics. Mixed esters are also used as rheological modifiers, for example in automotive paints to permit metal flakes to orient, which improves finish and drying times. Microcrystalline cellulose is also marketed as a dietary food additive and in pharmaceutical preparations.
The full potential of cellulose and cellulose products has not been fully exploited, partially due to the historical shift towards petroleum-based polymers from the 1940""s onwards, and also by the limited number of common solvents in which cellulose is readily soluble. Traditional cellulose dissolution processes, including the cuprammonium and xanthate processes, are often cumbersome or expensive and require the use of unusual solvents, typically with a high ionic strength and are used under relatively harsh conditions. [Kirk-Othmer xe2x80x9cEncyclopedia of Chemical Technologyxe2x80x9d, Fourth Edition 1993, volume 5, p. 476-563.] Such solvents include carbon disulfide, N-methylmorpholine-N-oxide (NMMO), mixtures of N,N-dimethylacetamide and lithium chloride (DMAC/LiCl), dimethylimidazolone/LiCl, concentrated aqueous inorganic salt solutions [ZnCl/H2O, Ca(SCN)2/H2O], concentrated mineral acids (H2SO4/H3PO4) or molten salt hydrates (LiClO4.3H2O, NaSCN/KSCN/LiSCN/H2O).
Physical and chemical processing methods for treating cellulosic resources are numerous. Chemical, enzymic, microbiological and macrobiological catalysts can be used to accelerate the process under conditions selected to be thermodynamically favorable to product formation. Chemical processes include oxidation, reduction, pyrolysis, hydrolysis, isomerization, esterification, alkoxylation and copolymerization. Chemical and enzymatic hydrolysis of cellulose is discussed in xe2x80x98The Encyclopedia of Polymer Science and Technologyxe2x80x99, 2nd Ed, J. I. Kroschwitz (Ed in Chief), Wiley (New York), 1985. Wood, paper, cotton, rayon, cellulose acetate, and other textiles are a few examples of the broad range of cellulosic materials.
With increasing industrial pollution and consequent governmental regulations, the need to implement xe2x80x98greenxe2x80x99 processes to prevent pollution and waste production and to utilize renewable resources is becoming increasingly prominent. The efficiency of existing methods for dissolving and derivatizing cellulose can be significantly improved by the availability of suitable solvents for refined and natural cellulose; an example is N-methylmorpholine-N-oxide (NMMO), used as a solvent for non-derivatizing dissolution of cellulose for the production of lyocell fibers. [http://www.lenzing.com.]
The use of ionic liquids as replacements for conventional organic solvents in chemical, biochemical and separation processes has been demonstrated. Graenacher first suggested a process for the preparation of cellulose solutions by heating cellulose in a liquid N-alkylpyridinium or N-arylpyridinium chloride salt, U.S. Pat. No. 1,943,176, especially in the presence of a nitrogen-containing base such as pyridine. However, that finding seems to have been treated as a novelty of little practical value because the molten salt system was, at the time, somewhat esoteric. This original work was undertaken at a time when ionic liquids were essentially unknown and the application and value of ionic liquids as a class of solvents had not been realized.
It has now been found that cellulose can be dissolved in solvents that are now described as ionic liquids that are substantially free of water, nitrogen-containing bases and other solvents. It has also been found that a wide and varied range of ionic liquids can be used to provide a greater control and flexibility in the overall processing methodology. It has further been found that cellulose-containing materials can be obtained from an ionic liquid solvent system without using volatile organic or other undesirable solvents in the process. These findings are discussed in the disclosure that follows.
A method for dissolving cellulose is contemplated. That method comprises admixing cellulose with a hydrophilic ionic liquid comprised of cations and anions in the substantial absence of water or a nitrogen-containing base to form an admixture. The admixture is agitated until dissolution is complete. The admixture is heated in some embodiments, and that heating is preferably carried out by microwave irradiation. The ionic liquid is molten at a temperature less than about 150xc2x0 C.
The cations of an ionic liquid are preferably cyclic and correspond in structure to a formula selected from the group consisting of 
wherein R1 and R2 are independently a C1-C6 alkyl group or a C1-C6 alkoxyalkyl group, and R3, R4, R5, R6, R7, R8 and R9 (R3-R9), when present, are independently a hydrido, a C1-C6 alkyl, a C1-C6 alkoxyalkyl group or a C1-C6 alkoxy group. The anions of the ionic liquid are halogen, pseudohalogen, or C1-C6 carboxylate. It is to be noted that there are two iosmeric 1,2,3-triazoles. It is preferred that all R groups not required for cation formation be hydrido.
A cation that contains a single five-membered ring that is free of fusion to other ring structures is more preferred. A cellulose dissolution method is also contemplated using an ionic liquid comprised of those cations. That method comprises admixing cellulose with a hydrophilic ionic liquid comprised of those five-membered ring cations and anions in the substantial absence of water to form an admixture. The admixture is agitated until dissolution is complete. Exemplary cations are illustrated below wherein R1, R2, and R3-R5, when present, are as defined before. 
Of the more preferred cations that contain a single five-membered ring free of fusion to other ring structures, an imidazolium cation that corresponds in structure to Formula A is particularly preferred, wherein R1, R2, and R3-R5, are as defined before. 
A 1,3-di-(C1-C6 alkyl)-substituted-imidazolium ion is a more particularly preferred cation; i.e., an imidazolium cation wherein R3-R5 of Formula A are each hydrido, and R1 and R2 are independently each a C1-C6-alkyl group or a C1-C6 alkoxyalkyl group. A 1-(C1-C6-alkyl)-3-(methyl)-imidazolium [Cn-mim, where n=1-6] cation is most preferred, and a halogen is a preferred anion. A most preferred cation is illustrated by a compound that corresponds in structure to Formula B, below, wherein R3-R5 of Formula A are each hydrido and R1 is a C1-C6-alkyl group or a C1-C6 alkoxyalkyl group. 
A solution comprised of cellulose in a molten hydrophilic ionic liquid solvent that is substantially free of water or a nitrogen-containing base is also contemplated. As above, the ionic liquid is comprised of cations and anions that are preferably those discussed above. A more preferred solution is comprised of cellulose dissolved in a hydrophilic liquid whose cations contain a single five-membered ring free of fusion to other ring structures, as discussed previously. A contemplated solution can be used as is to carry out further reactions on the cellulose such as acylation to form cellulose acetate or butyrate, or for regeneration.
A method for regenerating cellulose is also contemplated. That method comprises admixing a solution of cellulose in a molten hydrophilic ionic liquid solvent that is substantially free of water or nitrogen-containing base, or in a hydrophilic ionic liquid whose cations contain a single five-membered ring free of fusion to other ring structures with a liquid non-solvent for the cellulose that is miscible with the ionic liquid. The admixing causes the cellulose and ionic liquid to form solid and liquid phases, respectively. The solid phase is the regenerated cellulose that is preferably collected, as compared to being further reacted in situ. The ionic liquids used in this method are those discussed above.